Professor Harrison studied Natural Sciences at Christ’s College Cambridge before studying Medicine at St George's and then training in Infectious Diseases in London and Boston, USA. In Boston, funded by an NIH/Fogarty Award and an Infectious Diseases Society of America (IDSA) Fellowship, he trained in the laboratory of Stuart Levitz investigating host defence against Cryptococcus neoformans, the commonest cause of meningitis in Africa, and responsible for over 100,000 deaths per year and 10-20% of all HIV-related deaths.

On returning to the UK in 1997, he obtained an Advanced Training Fellowship from The Wellcome Trust, and set up a series of ongoing clinical studies, first in Thailand and subsequently through a network of collaborating centres across Sub-Saharan Africa. Laboratory work focuses on host immunity and novel immunotherapeutic approaches and understanding pathogen virulence and evolution. In addition, with Professor Amina Jindani, he has helped drive clinical studies to simplify and shorten the treatment of tuberculosis; and has led the hospital service and a research programme on multi-drug resistant TB. He remains actively involved in the academic supervision of trainees in Infectious diseases from across London.

Professor Harrison is also Professor of Medical Mycology at the MRC Centre for Medical Mycology at the University of Exeter, and a fellow of the Academy of Medical Sciences, and has served on guidelines panels for WHO, IDSA, the Southern African HIV Clinicians Society, and European Confederation of Medical Mycology (ECMM). He was a member of the MRC Infection and Immunity Board, and has served on many MRC panels in Global Health, the WHO expert panel on fungal infections, responsible for the WHO priority list of fungal pathogens, and the scientific advisory board of the Joint Programme Initiative on Antimicrobial Resistance (JPIAMR).

Professor Harrison’s main focus is a programme of research, including multiple phase II and pivotal phase III trials, driving advances in the diagnosis, prevention and management of HIV-associated cryptococcal meningitis, responsible for >100,000 deaths worldwide each year. The trials have demonstrated large reductions in mortality and driven rapid changes in global policy (defining all of the top 3 antifungal regimens recommended by WHO) and practice, global access to the required antifungal drugs, and large reductions in mortality in routine care.

Highlights include:

1. Development of Early Fungicidal Activity (EFA), the rate of clearance of infection derived from quantitative CSF cultures from serial lumber punctures as a novel, powerful phase II trial endpoint. The technique has been widely adopted and is established as an integral part of the pathway to evaluate new drugs and regimens.
   
   
2. Development of a screening and pre-emptive treatment strategy to prevent clinical cryptococcal disease in patients with advanced HIV disease. The REMSTART trial demonstrated a 28% mortality reduction in patients with advanced HIV disease, and led to a strong recommendation by WHO for screening, and wide-scale implementation. The current EFFECT trial is investigating enhanced antifungal pre-emptive therapy.
   
   
3. Co-investigator on development of a novel immunodiagnostic test for cryptococcal infection (IMMY CrAg test), enabling the screening strategy and earlier diagnosis of symptomatic cases. The test has revolutionized the diagnosis of cryptococcal disease.
   
   
4. Determination of host immune parameters favouring clearance of infection and survival in patients, and the benefit of adjunctive interferon-gamma. The work underpins current plans for a larger trial of ultra short-course interferon gamma for HIV-associated cryptococcal meningitis.
   
   
5. Chief Investigator, ACTA trial. The results drove rapid revision of WHO guidelines, and worldwide access to generic flucytosine. Implementation demonstrated a halving of acute mortality in studies from South Africa, Tanzania, Cameroon and Malawi.
   
   
6. Co-Chief Investigator, AMBITION-CM trial, pioneering use of single, high-dose Ambisome with an oral therapy backbone. The Ambisome regimen had fewer side effects, providing an even safer, easier to implement, and cost-effective regimen. Again, this was rapidly adopted by WHO and is being implemented, with Ambisome preferential pricing.
   
   
7. With Professor Amina Jindani, Professor Harrison has helped drive studies to simplify and reduce the duration of therapy for tuberculosis through the use of high dose rifamycins. A major phase III trial (RIFAQUIN) demonstrated for the first time that a once weekly continuation phase regimen, based on high dose rifapentine and moxifloxacin, was equivalent to standard short course chemotherapy. The RIFASHORT trial demonstrated that double-dose rifampicin (1200 mg/d) in a 4 month regimen is well tolerated and almost as effective as the standard 6 month regimen.
   
   
8. Approaches from the cryptococcal programme, and use of single high-dose Ambisome combinations, are now being directly applied by others in trials in other life-threatening fungal infections, including histoplasmosis and talaromycosis. Combination antifungal approaches are being tested in invasive candidiasis, and advocated for in invasive aspergillosis.

Recent:

1. Harrison TS, Walsh, TJ. Time to re-evaluate combination antifungal therapy for invasive aspergillosis and other invasive mycoses. Lancet Infect Dis 2026 published on-line Feb 5th

Selected:

1. A.E. Brouwer, A. Rajanuwong, W. Chierakul, G.E. Griffin, R.A. Larsen, N.J. White, Harrison TS. 2004. Combination antifungal therapies for HIV-associated cryptococcal meningitis: a randomized trial. The Lancet 363:1764-67.
   
   
2. J.N Jarvis, S.D. Lawn, M. Vogt, N. Bangani, R. Wood, Harrison TS. Screening for Cryptococcal Antigenaemia in Patients Accessing an Antiretroviral Treatment Program in South Africa. Clinical Infectious Diseases 2009; 48:856-62
   
   
3. Jarvis JN, Percival A, Bauman S, Pelfrey J, Meintjes G, Williams GN, Longley N, Harrison TS, Kozel TR. Evaluation of a Novel Point of Care Cryptococcal Antigen (CRAG) Test on Serum, Plasma and Urine from Patients with HIV-associated Cryptococcal Meningitis. Clinical Infectious Diseases 2011; 53:1019-23.
   
   
4. Jarvis JN, Casazza JP, Stone H, Meintjes G, Lawn SD, Levitz SM, Harrison TS*, Koup RA* (* equal contribution). The Phenotype of the Cryptococcus-specific CD4 Memory T-cell Response is Associated with Disease Severity and Outcome in HIV-associated Cryptococcal Meningitis. Journal Infectious Diseases 2013; 207:1817-28.
   
   
5. Jindani A, Harrison TS, Nunn A,, and Mitchison D, on behalf RIFAQUIN trial team. RIFAQUIN: A multicentre randomised controlled clinical trial to evaluate high dose rifapentine and a quinolone in the treatment of pulmonary tuberculosis. New England J Medicine 2014; 371:1599-1608.
   
   
6. Mfinanga S, Chanda D, Lesikari S, Guinness L, Bottomley C, Chijoka C, Masasi A, Kimaro G, Ngowi B, Kahwa A, Mwaba P, Harrison TS, Egwaga S, Jaffar S. Cryptococcal meningitis screening and community-based early adherence support reduces all-cause mortality among HIV-infected people initiating antiretroviral therapy with advanced disease: a randomised-controlled trial in Tanzania and Zambia. The Lancet 2015 385:2173-82.
   
   
7. Molloy SF, C. Kanyama, R.S. Heyderman, A. Loyse, MD,, and T.S. Harrison, for ACTA Trial Team. Antifungal Combinations for treatment of Cryptococcal Meningitis in Africa. New England J Medicine 2018 378:1004-17
   
   
8. Loyse A, Burry J, Cohn J,,, and Harrison TS. Leave no one behind: response to new evidence and guidelines for the management of cryptococcal meningitis in low-income and middle-income countries. Lancet Infectious Diseases. 2018 Oct 18. pii: S1473-3099.
   
   
9. Jarvis JN, Leeme TB, Molefi M,,, and Harrison TS.. Short-course high-dose liposomal amphotericin B for human immunodeficiency virus-associated cryptococcal meningitis: a phase 2 randomized controlled trial. Clin Infect Dis 2019; 68: 393–401.
   
   
10. Jarvis, JN., Lawrence, DS., Meya, DB..,,, and Harrison TS, for Ambition Study Group. Single-Dose Liposomal Amphotericin B Treatment for Cryptococcal Meningitis. N Engl J Med, 2022, 386, 1109-1120. doi:10.1056/NEJMoa2111904
    
    
11. Mashau RC, Meiring ST, Quan VC, et al, and the GERMS-SA. Outcomes of flucytosine-containing combination treatment for cryptococcal meningitis in a South African national access programme: a cross-sectional observational study. Lancet Infect Dis 2022; 22: 1365–73.
    
    
12. Mfinanga S, Kanyama C, Kouanfack C, et al, and the DREAMM Consortium. Reduction in mortality from HIV-related CNS infections in routine care in Africa (DREAMM): a before-and-after, implementation study. Lancet HIV 2023; 10: e663–73.
    
    
13. John L, Thomson D, Bicanic T, Hoenigl M, Brown A, Harrison TS, & Bignell E. Heightened Efficacy of Anidulafungin When Used in Combination with Manogepix or 5-Flucytosine against Candida auris In Vitro. Antimicrob Agents Chemother, 2023; 67, e0164522. doi:10.1128/aac.01645-22.
    
    
14. Samuels THA, Molloy SF, Lawrence DS, Loyse A, Kanyama C,,, Harrison TS, Jarvis JN, Gupta RK. Personalised risk-prediction tools for cryptococcal meningitis mortality to guide treatment stratification in sub-Saharan Africa: a prognostic modelling study based on pooled analysis of two randomised controlled trials. Lancet Glob Health. 2025;13(5):e920–30.
    
    
15. ECCMID 2023 Keynote Lecture: A Revolution in the Management of Cryptococcal Meningitis, delivered by Professor Tom Harrison. In this lecture, Professor Harrison describes a programme of work that has defined new prevention and treatment strategies for cryptococcal meningitis that have driven revision of  WHO guidelines, global access to the required drugs, and major reductions in mortality in implementation in routine care.

Current Grants

1. FailSafe (GAMRIF) Grant. (Co-Applicant, PI Bignell E) BAL2039 - A Novel anti-Cryptococcal treatment Evaluation (BALANCE-1) – pre-clinical optimization through in-vitro and animal model studies to inform phase 2 and 3 human trials. 2025-2026. Value £250,000.
   
   
2. Wellcome Trust: (Co-Applicant, PIs: Wake R, Govender N) COMBAT Candida - COMBination Antifungal Therapy for Candida Bloodstream Infections. 2025-2029. Value £6.5 Million.
   
   
3. MRC Programme Grant. (Co-Applicant, PI: Bignell E) Extending the utility and durability of antifungal agents via innovative treatment regimens that minimise drug resistance. 2024-2029. Value £3 million.
   
   
4. NIHR Global Health Group, on HIV-associated Fungal Infections. (Co-Applicant, PIs: Jarvis JN, Govender N) 2022-2026. Value £3 Million.
   
   
5. MRC/Wellcome Trust/DFID Global Health Trials scheme. (Co-Applicant, PIs: Molloy S, Govender N) Fluconazole plus flucytosine vs fluconazole alone for cryptococcal antigen-positive patients identified through screening: A randomised trial (EFFECT). 2021-2026. Value £4.57 million.
   
   
6. EDCTP. (Co-Applicant, Lead: DNDi) Improved flucytosine formulation for the treatment of meningitis in advanced HIV disease. 2020-2026. Euro 3.7 million.

Professor Harrison is involved in undergraduate and post-graduate teaching and training in Infectious Diseases and Global Health.

He lecturers regularly at St George’s; and widely at national and international meetings. For some years he led the South London Infectious Disease Training programme, and still co-leads academic training in Infection at St George's.

He has supervised and mentored many students to successful independent clinical-academic careers, several of whom have joined the group at St George’s or continue to collaborate, based in other institutions.